Polycarboxylic acids such as polyacrylic acid, copolymer of acrylic acid and maleic acid, etc. are used as is a dispersant, a chelating agent or a flocculant. It is generally known that, when these polycarboxylic acids are used as the chelating agent, the effect is influenced by the content of carboxyl groups and molecular weight and, furthermore, the larger the content of carboxyl groups and molecular weight, the better the effect to be exerted. When a chelating capacity of these polycarboxylic acids is measured by the method shown in the Examples described hereinafter, the polyacrylic acid exhibits a Ca ion sequestration capacity of about 300 mg/g. The copolymer of acrylic acid and maleic acid exhibits a Ca ion sequestration capacity of about 370 mg/g.
However, it is also well known that it is very hard to perform biodegradation of the polycarboxylic acid obtained by polymerizing a monomer having a vinyl group, such as polyacrylic acid, copolymer of acrylic acid and maleic acid, etc., using microorganisms. Therefore, for the purpose of obtaining a chelating agent which is useful as a builder for detergent, there has been made a trial of oxidizing polysaccharides as a natural polymer to obtain a polycarboxylic acid whose biodegradation is expected, or a salt thereof, heretofore.
JP-B-49-1281 discloses that a polycarboxylic acid is obtained from various polysaccharides by a two-stage oxidation method using periodic acid and chlorite or a single-stage oxidation method using a hypochlorite. However, an amount of carboxyl groups in the polycarboxylic acid obtained by the method disclosed in this gazette does not exceed two on average per monosaccharide unit, excluding a special example. As the special example wherein the content of carboxyl groups exceeds 2 per monosaccharide unit, Example 78 discloses the example of previously producing a monocarboxylated corn starch and further oxidizing the monocarboxylated corn starch and Examples 79 and 80 disclose the example using sodium arginate as polysaccharides. However, in these Examples, there is no description about the molecular weight of the resulting polycarboxylic acid and, therefore, these Examples do not suggest a polycarboxylic acid having a structure wherein both content of carboxyl groups and molecular weight are clearly defined. In the production method described in these Examples, a large amount of acetic acid is used when the formed dialdehyde derivative is oxidized with sodium hypochlorite to form a dicarboxylic acid. However, it is well known that, under these acidic conditions, polysaccharides are hydrolyzed and the molecular weight is drastically lowered. Besides, this method has a problem that the production process becomes complicated because the monocarboxylated starch is once produced and further oxidized. On the other hand, when arginic acid is used as a raw material, the method has an economical problem. JP-A-60-226502 discloses a method of oxidizing polysaccharides using a hypochlorite as an oxidizing agent under controlled reaction conditions to obtain a polycarboxylic acid. The polycarboxylic acid obtained by this method has sufficiently high molecular weight, but the content of the carboxyl unit is 81% at most and the content of carboxyl groups is less than 2 on average per monosaccharide unit. Therefore, it is not a polycarboxylic acid having a structure which simultaneously satisfies the content of carboxyl groups and the molecular weight.
JP-A-62-247837 discloses a method of oxidizing polysaccharides using a metal catalyst such as Pd in combination with an accelerator such as Bi to obtain a polycarboxylic acid. However, this method is characterized by oxidizing the reducing terminal end of polysaccharides and the content of carboxyl groups does not also exceed 2 on average per monosaccharide unit.
JP-A-4-175301 discloses a method of oxidizing polysaccharides in the presence of a hypobromite or a hypoiodite. This gazette does not disclose data which suggest the structure of a polycarboxylic acid as a product. However, since there is a description "it is understood in this specification that the term `dicarboxy polysaccharides or polycarboxy saccharides` used herein mean polysaccharides wherein almost all of C2-C3 diol functional groups are respectively converted into two carboxyl groups by means of ring opening" in the specification, the amount of carboxyl groups contained in the polycarboxylic acid of this gazette is considered to be 2 at most as an average value per monosaccharide unit.
JP-A-4-233901 discloses a method of oxidizing an enzyme hydrolysate of starch or dextrin with a hypochlorite or a periodate. In this gazette, there is no description about a structure of a polycarboxylic acid obtained after the oxidation reaction, particularly content of carboxyl groups. Also, a clear measurement example about the molecular weight is not disclosed. Even if the molecular weight is calculated based on a distribution of a glucoside unit disclosed in Example 5 assuming that a decrease in molecular weight does not arise at the time of the following oxidation reaction and three carboxyl groups are introduced per unit after the maximum reaction, a weight-average molecular weight is 915 at most. When using, as a raw material, starch or dextrin from which a higher molecular weight material may be obtained, as shown in Comparative Example 1, a Ca ion sequestration capacity of the resulting polycarboxylic acid is still low, e.g. 200 and 225, which shows that the content of carboxyl groups in the oxidation reaction product is low.
JP-A-08-500626 discloses a method of oxidizing potato starch suspended in carbon tetrachloride with dinitrogen tetraoxide, followed by oxidation with oxygen. The carboxyl group content of the polycarboxylic acid obtained by this method is very low, e.g. about 1 in average number of carboxyl groups per glucose unit.
As described above, those wherein the content of carboxyl groups exceeds 2 per anhydrous glucose unit and the weight-average molecular weight is not less than 2000 are not known with respect to a polycarboxylic acid obtained by oxidizing polysaccharides containing an anhydrous glucose as a constituent unit, or a salt thereof.